The present invention relates to spectrometry and in particular to a method and apparatus for producing a plasma by microwave power for heating a sample for spectrochemical analysis, for example by optical emission spectrometry or mass spectrometry.
It is known to excite a plasma to heat a sample for optical or mass spectrometry via an axial electric field (that is, axially of the plasma torch) using frequencies in the microwave region (typically 2455 Mhz). Examples of known microwave induced plasma (MIP) spectrometers, as discussed in U.S. Pat. No. 4,902,099 by Okamoto et al, employ a Beenakker cavity, which utilises a TM010 cavity, or a xe2x80x9cSurfatronxe2x80x9d. These suffer from the disadvantage that the plasma forms in the form of a ball or cylinder. Sample injected into such a plasma is heated directly by the microwave energy (principally by electron bombardment). This excitation is very vigorous and leads to the production of undesired interferences. Also, direct interaction between the microwave energy and a changing sample load can destabilise the plasma. A better approach is to form the plasma in the form of an annulus or hollow tube with the sample injected into the hollow core. The electrical energy is dissipated in the outer layer which consists of pure support gas, and the sample is heated from this outer layer via thermal conduction and radiation. This isolates the sample from the electrical energy and results in more gentle excitation.
The Okamoto et al patent discloses an MIP spectrometer which provides a plasma having improved characteristics. The Okamoto et al spectrometer uses an antenna having multiple parallel slots arranged around the circumference of a conducting tube which contains a plasma torch. The antenna is inside a cavity supplied with microwave power of TE01 mode.
The present invention in seeking to provide a relatively simple and inexpensive method and apparatus for producing a plasma for spectrometry which is in the form generally of a hollow cylinder, provides an alternative to the Okamoto et al arrangement.
Accordingly, in a first aspect the invention provides a method of producing a plasma for spectrochemical analysis of a sample comprising
supplying a plasma forming gas to a plasma torch,
applying microwave power to the plasma torch, and
relatively positioning the plasma torch to axially align it with a magnetic field maximum of the microwave electromagnetic field, wherein the applied microwave power is such as to maintain a plasma of the plasma forming gas for heating a sample entrained in a carrier gas for spectrochemical analysis of the sample.
In a second aspect, the invention provides a plasma source for a spectrometer comprising
microwave generation means for generating microwave power,
a waveguide for receiving and supplying the microwave power,
a plasma torch having passages for supply of respectively at least a plasma forming gas and a carrier gas with entrained sample,
wherein the plasma torch is positioned relative to the waveguide such that it is substantially axially aligned with a magnetic field maximum of the microwave electromagnetic field for excitation of a plasma of the plasma forming gas for heating the sample for spectrochemical analysis.
An axial magnetic field induces tangential electric fields which in turn induce circulating currents in the conducting plasma. These circulating currents induce a magnetic field which opposes the applied field and shields the core of the plasma region from the applied field. As a consequence, most of the current flows in the outer layer of the plasma creating the cylindrical shape required. The effect is known and is often referred to as the xe2x80x9cskin effectxe2x80x9d.
A considerable field strength is required in order to initiate and sustain the required plasma. This field strength is more readily achieved with a moderate sized microwave power source by use of a resonant cavity. Such a cavity stores energy at the resonant frequency and thus raises the peak field strength available for the same level of supplied microwave power. The degree to which this occurs is defined by the quality factor or Q of the cavity and Q""s greater than =10 have proven effective. A particularly preferred requirement of a cavity for this invention is that it produce a magnetic field maximum in an unencumbered region of space so that a plasma torch can be inserted at the magnetic field maximum. Many possible cavities exist and are described in appropriate microwave texts, for example xe2x80x9cMicrowave Engineeringxe2x80x9d by Peter A Rizzi ISBN 0-13-586702-9 1988 Prentice Hall.
A simple yet effective approach is to use a cavity formed from a length of waveguide short circuited at one end and fed with microwave power via a suitable iris from the other end. Such a cavity operates in the TE10 mode (where n is an integer that depends on cavity length). This also has the advantage of being readily fed with microwave power transmitted in the TE10 mode which is the most common and simplest way of transmitting microwave power along a waveguide. Cavities with a low Q offer the advantage of broad and therefore simple tuning. However they may not offer enough increase in magnetic field strength for optimum maintenance of the desired plasma. To this end magnetic field concentration structures may be employed within the cavity to further increase the peak magnetic field strength. In the case of a cavity formed by a waveguide which is short-circuited at one end, these can be conveniently provided by conducting bars (eg: metallic bars) placed in contact with each side of the inside wall of the cavity so as to reduce the cavity height in parallel alignment with the plasma torch. Rectangular bars may be used but preferably the height reduction is made more gradually for example by use of bars with a triangular cross-section with the apexes directed inwardly.
Alternatively a resonant iris may be provided within the waveguide and a plasma torch positioned relative to this iris such that the microwave electromagnetic field at the resonant iris excites the plasma.
Preferably the resonant iris is provided by a structure which defines an opening to provide the resonant iris by reducing a width and a height of the waveguide. The structure may be a metal section having a thickness dimension along the waveguide with the plasma torch accommodated within a through hole of the metal section which intersects the resonant iris opening.
According to a third aspect, the invention also provides a waveguide for a microwave induced plasma source for spectrochemical analysis of a sample,
wherein the waveguide is dimensioned to operate in the TE10 mode and includes apertures for accommodating a plasma torch, wherein the apertures are located such that in use a plasma torch located in the waveguide and extending through said apertures will be axially aligned with a magnetic field maximum of the microwave electromagnetic field.
For a better understanding of the invention and to show how it may be carried into effect, embodiments thereof will now be described by way of non-limiting example only, with reference to the accompanying drawings.